Steven asks if there might be 'something' else going on. Yes, there is. Climate.

Using silnylon in high humidity areas or where there are extreme temperature changes does cause issues. Not sure where you backpack, but bring one of your shelters to the West Coast of Washington or Vancouver and then re-post....; )

I mostly backpack in the damp NE coast of the US and I have seen my share of extreme temperature and humidity changes.

I have gone to bed on warm nights, waking to cold pouring rain and I use silnylon shelters 75% of the time.

I have also experienced the trekking pole creep that Javan mentions, but it was kind of obvious in my case.

I do admit that silnylon and any nylon for that matter will sag when wet. Nylon lines can sag as well.

Silnylon also has a lot of stretch, over tightening before I go to bed seems to be the best solution for me.

I suspect thick bungee would help as well, as long as it was thick enough to not give in a wind.

@Javan

I wish I could blame it on a slipping trekking pole, but mine are homemade jobbers that are constant length.

Sil isn't for me, but thankfully plenty of other people love it, so I shouldn't have any trouble finding a new loving home for my contrail once I get my next setup sorted out.

@Chad

Im sure most, including myself, would agree with you.

Edited by QNelson on 06/18/2010 17:26:51 MDT.

The Moment seems to overcome most of the "silnylon sag" problem with its hoop pole and shape design. I've seen is sag a bit in an all night rain but not the "15% stretch sag". If it did it wasn't readily apparent.

It seems both the moment and rainbow would, because of the extra poles, would resist sagging better than trekking pole supported models like the contrail, rainshadow, etc.

Why not use "sil-polyester"?? Dacron Polyester hardly stretches at all when wet, hence its use in most sails.

Can't very light dacron fabric be treated with silicone also? True, it doesn't have quite the strength of nylon but a light ripstop grid would help that issue, along with some care in use. Plus, it's a lot more UV resisant than nylon.

Just saying... :)_

P.S. I forgot to mention that my Moment has the top 1/2 of the canopy "home-coated" with a thin silicon layer which MAY be why my tent does not sag much.

Edited by Danepacker on 06/18/2010 22:38:25 MDT.

You could always try this:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=19526

I know it works for all my silnylon tents.

There is no consensus about this because there is no one fabric with consistent performance called "silnylon."
There are a number of manufacturers, and many of the sources are in Asia and unknown to consumers. Even for known sources, the quality varies from batch to batch, and the quality of the coatings is not consistent. As a do-it-yourselfer, I've found the only solution is to order a small quantity from a usually reliable supplier, test it, and if it is good, immediately place a larger order hoping to get the same batch.
Tent manufacturers have more resources, but are still subject to these variations. Added to that, the function of the fabric over time remains unclear. Coatings will "cure," and it is unclear what products have the best useful life.
Wrestling with these headaches, I've decided that for tents, dome or multiple hoop shapes with internal line tighteners are the only way to go with silicon coated nylon, even if it is the best quality I can find, and even if the tent has a full inner. This is consistent with the posters who have found less sagging issues with the hoop designs.
There is silicon coated polyester available, either in the form of the so-called "spintex," or tarps from Wilderness Experience and other companies. But either way, the expense is very high compared to nylon. Note that nylon has been an often used fabric for backpacking tents for decades, without sagging of the kind experienced by me and some of the posters on this thread. So perhaps it is the small denier, or very thin quality of what we call "silnylon" that contributes to this problem. Also, the arrival of single wall tents with minimal supporting structure aggravates the problem, making it much more likely that the sagging will get us wet.
Agree with Mat and David that when the weather cycles back and forth between moist and dry, tighteners and tensioners do not solve the problem in one operation. After all, a tent is primarily a place to sleep, not have to fumble with adjusting gear in the middle of the night. I have pitched a Tarptent in my yard, and watched it repeatedly alternate with the weather between taut and sagging for several days while I was repeatedly adjusting and tightening it. The swings between these extremes were so wide, that I knew it would perform poorly in strong winds when damp, and was afraid it would rip itself apart, or at least compromise its weatherproofness, if I continued to leave it up in dry sunshine. For that reason, I decided not to use the tent for serious trekking, where there is no hasty retreat. As said, I'm sticking with silnylon, but only what tests well, and only over a dome frame that minimizes the effect of sagging on comfort. Unfortunately, after much break testing, I do not expect carbon poles to withstand serious gales, and alloy poles are quite heavy, so that continues to be a problem. Roger Caffin's articles and many posts about his tents go a long way to addressing this issue, and it is not an insoluable problem. Just one that will require a good deal of thought and innovation with materials of consistently high quality. In the meantime, IMO, ample if not full interior netting, internal line tighteners and lots of well-supported headroom are a priority.
We live in a land fond of instant gratification, but this is one issue where some will have to put in a lot of sweat equity to get the kind of shelter many want. Unless you are one who is comfortable with just a tarp, in which case you have my sincere admiration; but I am one who, as the blues singer said, is 'built for comfort not for speed.'
See you in the mountains, Sam

Of the many joys of backpacking in Sierra last summer was the relatively low humidity. The tent stayed fairly taught throughout the night. As I live in the fine state of Washington, where the humidity is much higher on the western side of the Cascades, I've generally been less impressed by the performance of silnylon for the reasons mentioned. When it gets cold and wet, as it frequently does, the sagging can be fairly extreme.

This discussion is very good, I enjoy the input and observations of everyone.

I agree with Dirk and David, for environments that tend to be less humid and experience less rain, like the Sierra, silnylon shelters would seem to be much more suitable. I tend to take most of my trips in the rain soaked southern Appalachians, where the word "dry" is a relative concept, generally describing levels of dampness. Unless there is no precipitation, tarptents and silnylon shelters just seem much less suitable...and if there's no rain, why sleep under a shelter?